This invention relates to inverter apparatus, and more particularly to an improved type inverter apparatus capable of providing a polyphase output of variable voltage and variable frequency, which is adapted for driving an AC motor from a DC power source.
Known is a voltage-type inverter, one example of which having a three-phase output is indicated in FIG. 1 of the accompanying drawings.
This inverter comprises main thyristors M1 through M6 of a so-called reverse-conductive type and commutating circuits associated therewith, the latter circuits comprising auxiliary thyristors A1 through A6, commutation capacitors C1 through C3, and commutation reactors L1 through L3, respectively. Output terminals U, V, and W, from which the three-phase AC output of the inverter is obtainable, are provided between the main thyristors M1 and M2, M3 and M4, and M5 and M6, respectively.
This type of inverter apparatus has advantageous features in that a pair of main thyristors, such as M1 a and M2, connected in series across a DC power source E are commutated efficiently under the co-operation of a commutating capacitor C1 and a commutating reactor L1, by utilizing the reversed electric charge in the capacitor C1, and that in response to the load current commutated by the aid of the commutating reactor L1, the commutating capacitor C1 is charged to a voltage higher than the power source DC voltage, thereby increasing the commutation limit of the inverter apparatus automatically.
However, when this type of inverter apparatus is used for driving an AC motor, a wide variation in rotating speed of which requires a wide variation of the output voltage of the inverter, or requires an output of a sinusoidal waveform, such as obtainable by a pulse-width modulation, instead of the conventional rectangular waveform, so that a number of current chopping operations must be carried out within one cycle period of the AC output. In this case, when a main thyristor, for inslance, M1 is turned off by any reason, the load current cannot be interrupted instantaneously because of the inductance in the AC motor, but the load current is passed through the diode portion of the main thyristor M2. Although the load current can then flow through the main thyristor M1 when it conducts again, the immediately subsequent chopping of the main thyristor M1 is not possible because of the reversed charge in the commutating capacitor C1.
To eliminate this difficulty, it is required to render conductive the auxiliary thyristor A2 before the ignition of the main thyristor M1, thereby reversing the electric charge in the capacitor C1. Such a procedure, however, makes it impossible to utilize alternately the reversed electric charge in the commutating capacitor, which has constituted the first advantageous feature of this type of inverter.
Furthermore, in accordance with recent increase in the capacity and the output voltage of the inverter apparatus, the main thyristors M1 through M6 tend to be operated near their breakdown voltage, with the result that the commutating capacitors tend to be over-charged. As a result, the breakdown voltage of the auxiliary thyristors A1 through A6 must be increased by for instance connecting a number of identical elements in series, and the afore-mentioned variable commutation limit depending upon the variation in the load current, which constituted the second advantageous feature of this type of inverter apparatus, has now turned to be a disadvantageous feature.
Even in the case of operating the inverter apparatus at a comparatively low voltage, since the variation of the commutation limit depends much on power losses in the commutation capacitors and commutation reactors, it is difficult to measure the variation of the commutation limit precisely, and therefore the advantageous feature cannot be fully utilized in the design of the inverter.
In FIG. 2 of the accompanying drawings, there is indicated an example of a conventional current-type inverter wherein the output current is self-controlled within the own apparatus. In this example, there are provided six choppers CH1 through CH6 comprising main thyristors M1 through M6 of the reverse-conductive type, and commutating circuits, associated with the main thyristors, comprising auxiliary thyristors A1 through A6, also of the reverse-conductive type, commutating capacitors C1 through C6, and commutating reactors L1 through L6, respectively. Between the two choppers CH1 and CH2, CH3 and CH4, and CH5 and CH6, which are connected in series between the positive and negative buses of a DC power source E, are connected in series smoothing reactor pairs, LD1 and LD2, LD3 and LD4, and LD5 and LD6. Phase selecting and current circulating thyristors S1 through S6, are inserted between the choppers CH1 through CH6 and the positive and negative buses, respectively, and output terminals U, V, and W are provided intermediately of the smoothing reactor pairs, LD1 and LD2, LD3 and LD4, and LD5 and LD6.
In this type of the inverter apparatus, the main thyristors M1 through M6 in the choppers are on-off controlled at every electric angle of 60.degree., that is, at every 1/6 of one cycle period of the output frequency. In other words, the output frequency is determined by the on-off control of the main thyristors such that the current is commutated from one phase to the other, and the output current is controlled by changing the time ratio of the entire on-periods in each phase against the 60.degree.-chopper period.
Thus, in the conventional current-type inverter, the output frequency and output current can be controlled within the apparatus, itself and therefore the DC power source E may be of a constant voltage type which can be easily constituted by an ordinary diode thpe rectifier device which can preduce such an output from a conventional three-phase AC power line at a high efficiency and a high power factor.
Furthermore, this type of inverter apparatus has an advantageous feature of enabling to provide a fast current control as a result of the utilization of the chopper circuits.
However, in the inverter apparatus shown in FIG. 2, there is a possibility that the potential of the U-phase output terminal would be reduced to a value lower than that of the negative bus due to an induced voltage in the AC motor when, for instance, the main thyristor M1 and the phase selecting and current circulating thyristor S1 are both in off state. Therefore the commutation capacitor C1 is over-charged through the auxiliary thyristor A1, the latter feature requiring to increase the breakdown voltage of the auxiliary thyristors A1 through A6 than in the previous example. In addition, the provision of a commutating circuit for each chopper complicates the construction thus increasing the manufacturing cost of the inverter apparatus. Furthermore, as is apparent from FIG. 3, since the operation period of each chopper is only for an electrical angle of 60.degree., such an arrangement was found to be inefficient as to the commutating circuit.